Turn on the fixed position stacking context flag by default

content/browser/web_contents/web_contents_impl.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/web_contents/web_contents_impl.h"
 
 #include <utility>
 
 #include "base/command_line.h"
 #include "base/metrics/histogram.h"
@@ skipping 570 matching lines @@
   if (command_line.HasSwitch(switches::kMaxUntiledLayerHeight))
     prefs.max_untiled_layer_height =
         GetSwitchValueAsInt(command_line, switches::kMaxUntiledLayerHeight, 1);
 
   if (gfx::Screen::IsDIPEnabled()) {
     // Only apply when using DIP coordinate system as this setting interferes
     // with fixed layout mode.
     prefs.apply_default_device_scale_factor_in_compositor = true;
   }
 
-  prefs.fixed_position_creates_stacking_context =
-      command_line.HasSwitch(switches::kFixedPositionCreatesStackingContext);
+  prefs.fixed_position_creates_stacking_context = !command_line.HasSwitch(
+      switches::kDisableFixedPositionCreatesStackingContext);
 
   prefs.number_of_cpu_cores = base::SysInfo::NumberOfProcessors();
 
   content::GetContentClient()->browser()->OverrideWebkitPrefs(rvh, url, &prefs);
 
   return prefs;
 }
 
 NavigationControllerImpl& WebContentsImpl::GetControllerImpl() {
   return controller_;
@@ skipping 2519 matching lines @@
       browser_plugin_host()->embedder_render_process_host();
   *embedder_container_id = browser_plugin_host()->instance_id();
   int embedder_process_id =
       embedder_render_process_host ? embedder_render_process_host->GetID() : -1;
   if (embedder_process_id != -1) {
     *embedder_channel_name =
         StringPrintf("%d.r%d", render_view_host->GetProcess()->GetID(),
                      embedder_process_id);
   }
 }